The invention relates to a process for pretreating strips and foils of aluminum or aluminum alloys before lacquering, adhesive coating or laminating with plastic, and this by electrochemical oxidation in an electrolyte using alternating current.
Lacquered or laminated aluminum strips and foils are used especially for making foodstuff packaging, the aluminum being protected from corrosion by a coating of lacquer. For many forms of packaging it is sufficient for the lacquer or adhesive to be deposited on untreated aluminum. For demanding applications on the other hand e.g. the production of deep-drawn containers for pasteurizable or sterilizable contents, pretreatment of the strip or foils is necessary to achieve adequate adhesion.
A normal form of pretreatment is continuous degreasing followed by chemical conversion of the surface forming a so-called conversion coating. Most of these processes are based on a reaction of chromic acid, fluoric acid and phosphoric acid with aluminum, as a result of which the conversion coatings contain Cr.sup.VI compounds which are increasingly undesirable or even suspect in use with foodstuff packaging. Solutions permitting the formation of Cr.sup.VI -free conversion coatings are already known; the quality of these coatings with respect to adhesion and corrosion resistance is however still inadequate, especially for sterilizable forms of packaging.
It is known from German Patent No. 17 71 057 that continuous pretreatment of a strip of aluminum prior to lacquer coating is possible by passing the strip through an aqueous, sulfuric acid bath containing electrodes at a temperature between 50.degree. C. and the boiling point of the bath, and oxidizing the said strip electrochemically using alternating current. In that process an oxide layer is formed on the aluminum strip when, due to the alternating current, the aluminum is made the anode, and that layer is for the greater part redissolved again when the aluminum is made the cathode. As such the dissolution does not take place uniformly, but locally so that craters are formed in the very thin (approx. 0.05 m) oxide layer. The widespread pitting of the oxide layer is one reason for the good bonding of coating materials to the aluminum strip, especially lacquer coatings.
A further improvement in lacquer adhesion is achieved using the process described in German patent publication No. DE-OS 33 25 802 in which the oxidation of an aluminum strip is performed with 3 phase alternating current, one phase of which is transformed to dc, in such a manner that the strip passes three electrodes in succession. Of these electrodes at least one, preferably the middle one, is connected to the dc phase, the other two to the three phase supply.
The disadvantage of the known oxidation processes using acidic media is the relatively high concentration of sulfuric acid viz., 15-20%, and the operating temperature of at least 80.degree. C. which must be maintained otherwise a significant loss in adhesion results.
If the oxidation is performed in alkaline electrolytes e.g. containing hydroxides, carbonates and phosphates at concentrations below 1 weight percent and at temperatures below 50.degree. C., oxide layers providing good adhesion can be obtained. These alkaline processes, however, demand exact regulation of the pH value, temperature and concentration of constituents, which is very difficult with increasing concentration of aluminum in solution. Even small deviations from the correct value can suddenly lead from good to poor adhesion properties.
In addition, the use of silicate-containing electrolyte systems is disadvantageous because a silicate deposit is built up on the aluminum surface with the effect that the following procedure of electrochemical oxidation is imperfect in that the oxidized surface is disturbed. This causes a bad adherence of the lacquer during application and/or afterwards because the bad oxidized metal surface parts have the tendency to corrode within a short time. This is particularly undesirable where one thin layer coating of lacquer is applied, as for example, for foodstuff packaging in the range of 10 grams lacquer per square meter aluminum strip or foil.
The object of the present invention is to develop a process for pretreating aluminum strips or foils, prior to lacquering, coating with adhesive or laminating with plastic, using electrochemical oxidation and an electrolytic bath which operates at lower concentrations and temperatures than the baths used up to now, provides the same or better adhesion and is less aggressive. Furthermore the process should employ non-toxic constituents which will be non-hazardous to the environment i.e. no undesirable fumes resulting from decomposition and no contamination of waste water systems, and will leave no substances in the conversion coating which are damaging to the health.